Two-degrees-of-freedom (2DoF) composite panels are used in aircraft jet engines to attenuate noise. In general, 2DoF structures use a porous septum to separate two chambers. It is the sonic resonance between these two chambers, attenuated by the septum, that converts noise from the engine into heat.
The chambers in the panel are typically made out of honeycomb material. The honeycomb material is commonly made from an aluminum foil or paper material and comprises an array of closely-packed hollow channels that are typically hexagonal, square or circular in shape. The acoustic impedance of the septum, combined with the height of the honeycomb, determines which frequencies of noise are absorbed and to what degree.
One type of available 2DoF honeycomb construction contains a septum inserted into the cell of the honeycomb, which is then bonded in place either mechanically or with an adhesive. (US Patent Application No. 20080251315.)
Another type of 2DoF honeycomb construction is made from two pieces of honeycomb bonded together with a structural septum in between the honeycomb pieces. (U.S. Pat. No. 4,294,329.) The structural septum can be made out of materials such as perforated aluminum and porous fiberglass. The septum in this construction is a hard layer that attenuates noise by the transfer of stress between the two layers of honeycomb.
Both of these 2DoF constructions are difficult to fabricate and involve specialized equipment. In addition, nonlaminar air flow through a perforated 2DoF septum construction (e.g. perforated fiberglass) can result in poor acoustic performance.
Laminar air flow through a soft septum, such as cloth, offers better acoustic performance than does nonlaminar air flow through a perforated hard septum. A composite panel containing a soft septum also results in good mechanical properties such as flatwise tensile strength and plate shear strength. Therefore, it is desirable to have a 2DoF composite panel with a soft woven septum separating two honeycomb panels.
It is difficult to produce a strong bond between a soft cloth septum such as PET and a honeycomb panel using aerospace grade epoxy adhesives. There are several known ways to promote adhesion with soft cloth such as, for example, corona or oxygen plasma treatment, chemical etching, and application of primer coatings. However, both corona and oxygen plasma treatments are perishable and quickly lose effectiveness. Furthermore, it would be difficult to objectively prove that a roll of cloth had been treated before use in aircraft.
Chemical etching with hot sodium hydroxide has several drawbacks. Chemical etching is a slow process, and it is difficult to fully rinse all of the caustic from the cloth after treatment. Residual sodium hydroxide could disrupt the cure of the adhesive, creating a latent flaw in the composite panel. In addition, residual sodium hydroxide could soften or corrode structures adjoining the composite panels in places that cannot be inspected.
Primer coatings can be applied to the cloth prior to adhesion, but they tend to fill in and the block the open area of the cloth, making acoustics performance unpredictable.
Prior attempts to use a soft septum in a 2DoF honeycomb construction have been unsuccessful. It is difficult to adhere a soft septum to honeycomb and still have a structure with good mechanical properties. Accordingly, there is a need for a new system of a composite panel with a soft septum that avoids the above-described problems in the prior art.